Recently, easy handling of a color image has been attained even in ordinary offices, as well as in offices of special fields, such as computer-graphic designing, when popularized are electronic apparatuses on a basis of color images. When a color image produced by a personal computer (PC) or digital still camera is transferred by electronic mail (E-mail), so that the color image is stored in a recording medium such as a hard disk, a floppy disk, or a recording medium of a digital still camera (for example, memory stick® or smart media®), and displayed on an image display device by using the data in the recording medium, the image display device generally has had difficulty in color investigation of the color image, because the sender and the receiver of the color image cannot match their colors. Color management has been contrived as a solution for the problem, and is drawing attention.
The color management equalizes differences in colors between each image display device by utilizing a common color space. In other words, color management attains an accordant expression of colors by describing all colors in a single color space, in which coordinates corresponding to the colors are accorded between colors of different devices. This is based on an idea that colors described by the same coordinates in a single color space have the same expression.
One color management method commonly used today corrects the differences between each device with a CIE-XYZ color space as the color space, and by using XYZ tristimulus values that are internal descriptive coordinates in the CIT-XYZ color space. Japanese Unexamined Patent Publication, Tokukaihei No. 11-134478 (published May 21, 1999), discloses a technology in which accordant color expression is achieved by this method.
FIG. 15 explains an environment in which each PC display image is viewed via the color management. A display image 152, which was displayed on a display device 151 of a sending PC, is displayed on a display device 153 of a receiving PC.
Generally, there is a difference between the sending PC and the receiving PC as to how much the color reproduction characteristics are changed with passage of time. Moreover, the transferred image is displayed on display devices with different color reproduction characteristics, respectively, and under a condition in which an image viewing condition and an environment, such as illumination light, are varied.
In FIG. 15, however, illumination light 154 of the sender and illumination light 155 of the receiver are varied. In this case, expression of an image is varied in accordance with the variation in illumination light. Thus, an isochromatic sensation cannot be attained, even though the image has the isochromatic color under one of the illumination lights. Moreover, when the display device is, for example, a transmission type liquid crystal display device (a transmission type LCD), long-time continuous use of the device causes a change in color filter characteristics with passage of time, and changes in a back light source due to a change in surrounding temperature and passage of time. This leads to changes in brightness and color of the displayed objects. Therefore, it has been a problem that long-time continuous use, which causes a far greater change in the expression of the image, cannot have an isochromatic sensation.
Image display devices equipped with a reflection type liquid crystal display device (a reflection type LCD) have been popularized for portable information terminals and PCs. Because its display theory is based on reflection of external light (light from the exterior of the device) such as illumination light, the reflection type LCD is affected more significantly by the external light in terms of display quality, compared to the transmission type LCD. Broadly speaking, two reasons, which are discussed below, explain the above characteristics of the reflection type LCD.
A first reason involves the fundamental theory of the reflection type LCD for displaying an image, understood with reference to FIG. 16.
FIG. 16 shows an example in which a reflection type LCD is used as a display device of a notebook-sized PC. Illumination light A strikes reflection type LCD 161, which emits light modulated by a color filter or a liquid crystal. The emitted light is denoted B. A user 162 of the image display device views the emitted light B. Needless to say, a change in the emitted light B gives the user 162 a feeling that image quality is changed.
FIGS. 17A–17E show examples of various characteristics, in which the horizontal axis is wavelength of light, and the vertical axis is relative intensity of light. For example, if the illumination light A in FIG. 16 had characteristics shown in FIG. 17A, while light modulation characteristics of the reflection type LCD are characteristics shown in FIG. 17B, the emitted light B in FIG. 16 would be described as shown in FIG. 17C, that is, as a product of the characteristics shown in FIG. 17A and those shown in FIG. 17B, where the product is calculated per wavelength. The emitted light B in FIG. 16 is changed as shown in FIG. 17E in accordance with a change of the illumination light A in FIG. 16 to be as shown in FIG. 17D. Moreover, the above-mentioned characteristics are discussed with reference to FIG. 18. FIG. 18 is a CIExy chromaticity diagram, in which indicates chromaticity coordinates of the emitted light B in FIG. 16 described in FIG. 17C. Meanwhile, x in FIG. 18 indicates chromaticity coordinates of the changed emitting light B shown in FIG. 17E. Thus, the user 162, viewing the emitted light B feels that the displayed color is changed from to x simply by a change in the illumination light A, and thus senses that the image quality is changed.
A second reason involves human vision, which has characteristics to adapt to color of illumination light. Therefore, the reflection type liquid crystal, which displays an image by using illumination light as its lighting source, needs to take the adaptation characteristics of humans in consideration for displaying. Otherwise, a′ change in the image quality is noticed.
The change of the displayed color from to x in FIG. 18 is due to the change of the illumination light A from the light with the characteristics shown in FIG. 17A to the light with the characteristics shown in FIG. 17D. In most cases, the user 162 of FIG. 16 views the LCD under this illumination. In other words, he adapts to the illumination light A. A change of the illumination light in FIG. 17A into that in FIG. 17D indicates that the adaptation condition is also changed.
Thus, a human cannot sense precisely the change of the displayed color from to x in FIG. 18, which is caused by the change in the illumination light. For example, the user 162, who senses a color of in FIG. 18 under the illumination light in the FIG. 17A, feels that a color of x in FIG. 18 looks like a color of in the FIG. 18, because the adaptation condition is varied with a change of the illumination to be as shown in the FIG. 17D.
In any case, a change in the illumination (external light) gives the user a sensation that the image quality of the LCD varies.